This application claims the benefit of British Patent Application No. 9914454.4, filed Jun. 21, 1999.
This invention relates to a heater unit and is particularly, although not exclusively, concerned with a heater unit which is adapted to be installed in the ducting of an air handling system.
Air handling systems commonly include equipment for heating, cooling, humidifying and otherwise conditioning the air passing through the system. Heating is commonly achieved by a heater unit comprising a peripheral frame across which heating elements extend in order to heat the air passing over them. The frame is of the same general dimensions as the surrounding ducting and serves as part of that ducting for the air passing over the heating elements. The elements themselves may be heated electrically or by steam.
It is known to install gas fired heaters in air handling systems. However, such heaters are usually substantially smaller in flow cross-section than the ducting itself and so the air has to be fed to, and drawn from, the unit heaters by means of convergent and divergent ducting. This restriction in flow cross-section has the effect of increasing the load on the fan drawing air through the system. Nevertheless, the use of gas-fired burners (or indeed burners using other forms of fuel) tends to be more efficient than electric heaters or those using steam.
Furnaces or space heaters are known from, for example, U.S. Pat. No. 4,974,579 (Shellenberger), U.S. Pat. No. 5,301,654 (Weber) and U.S. Pat. No. 5,775,318 (Haydock), in which air to be heated passes over a plurality of heater tubes. Each heater tube has a burner at an inlet end and an exhaust end which discharges to a flue. Thus burning of a fuel, such as gas, takes place in the heater tubes, and heat from the combustion gases is transferred to the air to be heated.
Such furnaces or space heaters are not suitable for mounting in an air handling system. This is because they are bulky, and so cannot be fitted in the relatively narrow space which tends to be allowed for electric or steam heater units. Furthermore, the known furnaces and space heaters do not provide a straight path for the air to be heated, and so they cannot be mounted in line in an air heating system.
According to the present invention there is provided a heater unit comprising a peripheral frame having an inlet and an outlet disposed opposite each other. A plurality of heater tubes extend across the frame, each heater tube having an inlet end and an outlet end. A respective burner unit is disposed at the inlet end of each heater tube, and a common manifold unit is provided into which opens the outlet end of each burner tube.
Such a heater unit can be installed in an air handling system, with the peripheral frame serving as part of the ducting of the system. Air flowing through the frame can thus be heated efficiently as it passes over the heater tubes.
Preferably, the heater tubes extend substantially across the full width of the peripheral frames. The tubes may include one or more reverse bends in order to increase the effective length of the heater tube within the frame. The frame is preferably rectangular, in which case the inlet and exhaust ends of the heater tubes may be provided on the same side of the frame.
A fan may be provided to draw the combustion products of the burner units through the heater tubes, and this fan may be positioned to extract these combustion products from the manifold.
The diameter of each heater tube may be greater at the inlet end than at the exhaust end. For example, where each heater tube has one or more reverse bends, the length of the heater tube approximately up to the first reverse bend may be of a larger diameter than the remainder of that heater tube.
It is preferable for the length of the heater unit, in the direction of air flow through the peripheral frame, to be kept as small as possible. This length is therefore preferably not more than 50% and more preferably not more than 25% of the length of the longer side of the peripheral frame, when the frame is rectangular.
Another aspect of the present invention provides air handling equipment including ducting and a heater unit as defined above, the peripheral frame of the heater unit comprising part of the ducting.
In accordance with a third aspect of the present invention, there is provided a method of modifying air handling equipment having a heater which includes a peripheral frame forming part of the ducting of the air handling equipment, the method comprising removing the existing heater and replacing it with a heater unit as defined above.